Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2011
Publication Date: 3/29/2011
Citation: Schmelz, E.A., Kaplan, F., Huffaker, A., Dafoe, N.J., Vaughan, M.M., Ni, X., Rocca, J.R., Alborn, H.T., Teal, P.E. 2011. Identity, regulation, and activity of inducible diterpenoid phytoalexins in maize. Proceedings of the National Academy of Sciences. 108(13):5455-5460. Interpretive Summary: Many plants respond to pathogens and insect attack by increasing the production of chemical defenses to protect against further damage. In response to pathogens, rice plants produce a complex array of diterpenoid phytoalexins which are directly antimicrobial and limit the spread of disease. Numerous biochemical defenses have been characterized in corn plants; however, to date functionally analogous diterpenoid phytoalexins have not been demonstrated in this crop. By examining the interactions of corn, stem boring insects and pathogens, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that ent-kaurane related diterpenoid phytoalexins rapidly accumulate in stalks. The compounds, termed kauralexins, are partly regulated by interactions of the defense-related plant hormones jasmonic acid and ethylene. Kauralexin production is highly localized and occurs in a diverse collection of maize varieties. At physiologically relevant concentrations, kauralexins significantly inhibit the growth of important fungal pathogens, such as anthracnose stalk rot (Colletotrichum graminicola), and are European corn borer (Ostrinia nubilalis) larvae antifeedants. This new finding highlights a previously unrecognized defense mechanism in maize against both insects and pathogens. Importantly, future elucidation of the genetic and molecular regulation of kauralexin biosynthesis should enable improved maize resistance to pests and pathogens.
Technical Abstract: Phytoalexins constitute a broad category of pathogen and insect-inducible biochemicals that locally protect plant tissues. Due to their agronomic significance, maize and rice have been extensively investigated for their terpenoid-based defenses which include insect-inducible monoterpene and sesquiterpene volatiles. Rice also produces a complex array of pathogen-inducible diterpenoid phytoalexins. However, despite the demonstration of fungal-induced ent-kaur-15-ene production in maize over 30 years ago, the identity of functionally analogous diterpenoid phytoalexins in maize has remained elusive. In response to stem attack by the European corn borer (Ostrinia nubilalis) and fungi, we observed the induced accumulation of six ent-kaurene-related diterpenoids, collectively termed kauralexins. Isolation and identification of the predominant Rhizopus microsporus induced metabolites revealed ent-kaur-19-al-17-oic acid and the novel analog ent-kaur-15-en-19-al-17-oic acid, assigned as kauralexin A3 and B3, respectively. Encoding an ent-copalyl diphosphate synthase, fungal-induced An2 transcript accumulation precedes highly localized kauralexin production which is detectable within 16 hours and can eventually exceed 100 'g g-1 FW. Pharmocological applications of jasmonic acid and ethylene also synergize the induced accumulation of kauralexins. Occurring at elevated levels in the scutella of all inbred lines examined, kauralexins appear ubiquitous in maize. At concentrations as low as 10 'g ml-1, kauralexin B3 significantly inhibited the growth of the opportunistic necrotroph R. microsporus and the causal agent of anthracnose stalk rot (Colletotrichum graminicola). Kauralexins also exhibited significant O. nubilalis antifeedant activity. Our work establishes the presence of diterpenoid defenses in maize and enables a more detailed analysis of their biosynthetic pathways, regulation and crop defense function.